Dual frequency antenna



Aug. 4, 1970 DUAL FREQUENCY ANTENNA F'il-ed Dec. 20, 1968 5 Sheets-Sheet 1 Maurice L. Fee,

INVENTOR.

ATTORNEY.

M. L. FEE 3,523,297

. Aug. 4, 1970 Filed Dec. 20, 1968 Fig. 3.

Aug. 4, 1970 M. 1.. FEE 3,523,291

DUAL FREQUENCY ANTENNA Filed Dec. 20, 1968 5 Sheets-Sheet 3 Fig. 5.

Trunverse Electric (TE) Mode in Hollow- Fin Trough Guide TEM Mode in Hollow-Fin Trough Guide Maurice Fee,

INVENTOR.

PM 9M ATTORNEY.

' Aug. 4, 1970 M. L. FEE 3,523,291

DUAL FREQUENCY ANTENNA Filed Dec. 20, 1968 5 Sheets-Sheet 4 20 IO 0 Angle Degree '0 IO N Fig. 7.

Maurice L. Fee, INVENTOR.

ATTORNEY.

M. L. FEE

Aug. 4, 1970 DUAL FREQUENCY ANTENNA 5 Sheets-Sheet 5 Filed Dec. 20, 1968 mm m nm a Maurice L. Fee, INVENTOR.

PMW. 7M

ATTORNEY.

United States Patent O U.S. Cl. 343-771 7 Claims ABSTRACT OF THE DISCLOSURE The apparatus of the present invention employs integrated radiating structure for two frequencies to provide a dual frequency antenna having a low frequency bandwidth which is greater than that obtainable with a slotted waveguide array and a high frequency bandwidth comparable to the bandwidth of a slotted waveguide array. The dual frequency antenna has an isolation between the two frequency channels thereof that may be made greater than that normally obtainable in dual frequency multimode antennas. The different frequency elements of the antenna may be oriented in a manner to radiate in space quadrature and thereby enhance the isolation between the two channels. The antenna is relatively simple to design and fabricate and may be used in a planar array configuration without exceeding a one wavelength spacing between radiating elements of the same frequency channel thereby minimizing the generation of undesirable secondary lobes in the array pattern.

BACKGROUND OF THE INVENTION There are several types of contemporary dual frequency microwave antennas. In one contemporary dual frequency antenna, a common reflector is fed with dual frequency feeds. This type of antenna is usually not very efficient, especially when there is a large separation in frequencies. The low efficiency is the result of the aperture blockage at the high frequencies by the low frequency feed.

Another contemporary dual frequency antenna comprises two waveguide arrays, each designed for a different frequency with their respective waveguide radiating elements interlaced. A dual frequency antenna made with interlaced waveguides does not have the aperture blockage problem associated with reflector antennas. It is difficult in most cases, however, to obtain a spacing of less than a free space wavelength between adjacent elements corresponding to the same frequency. A spacing equal to, or larger than a free space wavelength is undesirable because it gives rise to secondary maxima in the array pattern which are of the same order of magnitude as the main beam.

A third contemporary type of dual frequency antenna constitutes a slotted waveguide excited in two distinct modes with each mode at a different frequency. Although higher order mode dual frequency arrays have been successfully built, a different design problem peculiar to higher mode antennas is that of preventing mode conversion. If mode conversion occurs, the unwanted modes will propagate in the guide and cause pattern degradation.

SUMMARY OF THE INVENTION In accordance with the present invention, a dual frequency antenna array is provided that is composed of slotted hollow fin trough waveguide with obstacles located on the sides of the hollow center fin. The trough guide with obstacles located on the sides of the hollow center fin is the low frequency section of the antenna. The slotted hollow center fin, on the other hand, constitutes the high frequency section of the antenna.

3,523,297 Patented Aug. 4, 1970 In the operation of a single linear array, the slotted hollow fin in the center of the trough functions in a conventional manner as a waveguide slotted on the narrow side at half wavelength intervals. In the hollow fin trough guide, obstacles are placed on the side of the center fin to cause transverse-electric (TE) to transverse-electric-magnetic (TEM) mode conversion and hence radiation. Adjacent linear arrays in a planar array constructed in accordance with the invention are separated by less than one wavelength, thus avoiding pattern degradation due to spacing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a partially cutaway perspective view of a dual frequency planar array employing periodically asymmetric trough guide;

FIG. 2 shows a perspective view of a linear array of the planar array of FIG. 1 with feed attachments;

FIG. 3 shows a cross-sectional view of the right extremity of the linear array of FIG. 2, as shown in the drawing;

FIG. 4 shows a portion of a partially cutaway perspective view of an alternate linear array employing asymmetric trough guide;

FIGS. 5 and 6 illustrate transverse electric and transverse electric magnetic modes in a cross-sectional view of hollow fin trough waveguide;

FIG. 7 shows the E-plane high frequency pattern generated by the dual frequency linear array of FIG. 2; and

FIG. 8 shows the H-plane low frequency pattern generated by a dual frequency array of the type illustrated in FIG. 2.

DESCRIPTION Referring now to FIG. 1 of the drawings, there is shown a portion of a dual frequency planar array 10 composed of parallel linear dual frequency arrays 11, 12, 13, 14. Each of the dual frequency linear arrays 11-14 includes a trough section 16 which when adjacent to another trough section 16 may have common sides 17. In addition, high frequency waveguide 18 is disposed edgewise along the center portion of the base of the trough section 16 thereby to provide a hollow fin divider. The high frequency waveguide 18 may, ofcourse, have a common narrow side with the center portion of the base of trough sections 16 if deemed desirable in the fabrication process. The trough sections 16 together with the high frequency waveguide 18 disposed therein in the above manner constitutes hollow fin trough waveguide which provides the propagating structure for the low frequency wave of the dual frequency linear arrays 11-14. The high frequency waveguide 18, of course, provides the propagating structure for the high frequency wave of the dual frequency linear arrays 11-14. When used in the dual frequency planar array 10 of FIG. 1, the width of trough section 16 is preferably less than a free space wavelength at the high frequency to enable spacing of less than one wavelength between adjacent linear arrays 11-14 and thus avoid undesirable secondary lobes in the array pattern of the high frequency beam.

In general, the width of the high frequency waveguide 18, i.e. the heighth of the waveguide 18 in the trough section 16, is of the order of 50 percent of the width of the common sides 17. In addition, the high frequency waveguide 18 divides the trough sections 16 into three substantially equal portions. In order for the low frequency wave to generate a broadside beam from the dual frequency planar array 10, as will be hereinafter explained, conductive base blocks 20 are periodically disposed asymmetrically at the bottom of the trough section 16 between the side walls thereof and high frequency waveguides 18. The heighth of the base blocks 20 is of the order of one-half the width of the waveguide o 18 and the length equal to of the order of one-half wavelength at the low frequency. The purpose of the base blocks 20 is to effect a TB to TEM mode conversion essential forradiation of the low frequency wave. The

heighth of the base blocks 20 is sufficiently low so as not to interfere with radiation of the high frequency wave from the waveguides 18.

In the dual frequency planar array of FIG. 1, radiation of the high frequency wave is effected by means of slots 22 disposed in the top narrow wall of waveguides 18 as shown in the drawing, at intervals of one-half guide wavelengths for the high frequency wave. When the slots 22 are oriented transversely across the narrow wall of waveguide 18, it is necessary that alternate extremities of the slots 22 extend for greater distances into the broad walls of the waveguides 18 to effect radiation of the high frequency wave. Radiation elements of this type are described and claimed in copending application for patent entitled, Waveguide Side Wall Slot Radiator, Application Ser. No. 774,837 filed Nov. 12, 1968, by Maurice L. Fee and assigned to the same assignee as the present application for patent. An alternate form of high frequency radiation element is to employ angled slots as shown in connection with FIG. 3 of the drawings.

Referring to FIG. 2 of the drawings, there is shown a standing wave dual frequency linear array 11, 12, 13, or 14 of FIG. 1 together with inputs for the low and high frequency waves. In particular, the left extremity of high frequency waveguide 18, as shown in the drawing, is extended and a flange 23 attached thereto to provide a connection to a high frequency system, not shown. Referring to FIG. 3, there is shown a cross-sectional view of the right extremity of the linear array of FIG. 2, wherein waveguide 18 at this extremity is terminated by a transverse plate 24.

The low frequency section of the dual frequency linear arrays 11-14 is fed through a coaxial connector 25 mounted on a transverse plate 27 at the extremity of the trough section 16 opposite from flange end of waveguide 18 with a center conductor 26 extending through to the shorting plate 24 of waveguide 18. A transverse plate 28, FIG. 2, is disposed across the trough section 16 at the extremity thereof opposite from coaxial input connector 25 to provide a short circuit for the low frequency array. In addition, a taper 29, FIG. 3, is provided at the end of waveguide 18 adjacent coaxial connector 25 for impedance matching the coaxial connector 25 to the hollow-fin trough guide 11-14. The taper 29 gradually increases in heighth as the end of the high frequency waveguide 18 is approached over a distance comparable to the width of a broadwall thereof. In operation, low frequency energy fed through the coaxial input connector 25 establishes an electric field between the broad walls of waveguide 18 and the sidewalls of trough section 16. This field intensity pattern of the low frequency electric field is generally known as a transverse electric (TE) mode, which TE mode is illustrated in FIG. 5. Inasmuch as the electric fields on opposite sides of the waveguide 18 are equal and opposite, there is no radiation from the aperture when the TE mode is being propagated in a hollow fin trough waveguide array 11-14. As the low frequency wave propagates along the trough section 16, however, the base blocks 20 progressively convert the TE mode to a transverse-electric-magnetic (TEM) mode by blocking the TE wave on first one side and then the other. The resulting TEM wave is not balanced, whereby radiation occurs from the aperture of the trough waveguide section 16 in the manner illustrated by the electric field intensity pattern of FIG. 6. An H-plane low frequency pattern generated by the linear array of FIG. 2 is illustrated in FIG. 8 of the drawings.

Radiation from the high frequency slot array provided by the slots 22 in the high frequency waveguide 18 is generally conventional except to the extent that the resulting pattern is modified by the trough section 16 and 4 base blocks 20. By restricting the height of the base blocks 20 to less than half the width of waveguide 18, modification of the high frequency pattern is minimized. An E-plane high frequency pattern of the linear array of FIG. 2 is illustrated in FIG. 7 of the drawings. The standing-wave linear array 11-14 of FIGS. 2 and 3 may be converted to a traveling-wave linear array if desired by providing appropriate terminations for the high and low frequency sections. To convert the high frequency section to a traveling-wave array, a terminating impedance (not shown) would be placed in the waveguide 18 adjacent the shorting plate 24. To convert the low frequency section to a traveling-wave array, terminating impedances such as resistive cards (not shown) would be placed on each side of high frequency Waveguide 18 adjacent transverse plate 28.

Referring to FIG. 4 there is shown a dual frequency linear array 30 adapted to generate a low frequency pattern having an end-fire beam rather than a broadside beam as in the case of the linear array of FIG. 2. The linear array 30 includes the trough section 16 and the high frequency waveguide 18 as before. A conductive base block 32 is, however, disposed continuously along trough section 16 between one side wall thereof and a broadwall of waveguide 18. Base block 32 has a height equal to of the order of one-half the width of a broadwall of waveguide 18. Angled slots 34 in the narrow wall of waveguide 18 constitute radiation elements for the high frequency wave. The linear array 30 may be fed in the same manner as the linear array of FIG. 2. As before, it is necessary for the TE to TEM mode conversion apparatus, i.e. continuous base block 32, to remain clear of the narrow wall of waveguide 18 containing the radiating slots 34 to minimize the modification of the high frequency pattern.

What is claimed is:

1. A dual frequency linear spray for high and low frequencies, said array comprising a length of high frequency rectangular waveguide having an input connection at one extremity thereof; means included in a narrow wall of said waveguide for providing radiation elements along a portion of said length of high frequency waveguide; a conductive trough section having first and second wide walls and a base disposed about said waveguide concurrent with said portion thereof with the center portion of the inside surface of said base in contact with the remaining narrow wall of said waveguide thereby to form a hollow fin trough waveguide; means coupled to said hollow fin trough waveguide at an extremity thereof opposite from said one extremity of said high frequency waveguide for launching a low frequency TE mode wave therein; and means disposed in said hollow fin trough waveguide for converting said TE mode to a TEM mode thereby to effect radiation of said low frequency wave.

2. The dual frequency linear array for high and low frequencies as defined in claim .1 wherein said means disposed in said hollow fin trough waveguide for converting said TE mode to a TEM mode constitutes base blocks disposed periodically along said hollow fin trough waveguide on alternate sides of said high frequency waveguide whereby a low frequency broadside beam pattern is generated by said array.

3. The dual frequency linear array for high and low frequencies as defined in claim .1 wherein said means disposed in said hollow fin trough waveguide for converting said TE mode to a TEM mode constitues a continuous base block disposed along said hollow fin trough waveguide on one side of said high frequency waveguide whereby a low frequency end-fire beam pattern is generated by said array.

4. A dual frequency planar array comprising a flat conductive sheet; a plurality of lengths of high frequency rectangular waveguide disposed in parallel uniform intervals with one narrow wall thereof common with said flat conductive sheet; means disposed in the respective remaining narrow walls of said high frequency rectangular Waveguides for providing radiating elements disposed substantially at intervals of one-half guide wavelength; flat conductive strips disposed normal to said flat conductive sheet on each side of and equidistant from said respective high frequency waveguides, the height of said strips being greater than that of a broadwall of said high frequency waveguide thereby to form hollow fin trough waveguides therewith; means coupled to said respective hollow fin trough waveguides for launching a low frequency TE mode wave therein; and means disposed in said respective hollow fin trough waveguides for converting said TE mode to a TEM mode thereby to effect radiation of said low frequency wave.

5. The dual frequency planar array as defined in claim 4 wherein said parallel uniform intervals are less than one free space wavelength at said high frequency.

6. The dual frequency planar array as defined in claim 4 wherein said means disposed in the respective remaining narrow walls of said high frequency rectangular waveguides for providing radiating elements disposed substantially at intervals of one-half guide wavelength constitute slots oriented at an angle with the longitudinal axis of said waveguide thereby to control the radiation therefrom.

7. The dual frequency planar array as defined in claim 4 wherein said means disposed in the respective remaining narrow walls of said high frequency rectangular waveguides for providing radiating elements disposed substantially at intervals of one-half guide wavelength constitute transverse slots having alternate extremities extending greater distances into the broadwalls of said high frequency waveguide.

References Cited UNITED STATES PATENTS 2,943,325 7/1960 Rotman 343-772 X 3,015,100 12/1961 Rotman 343-772 3,193,830 7/1965 Provencher 343-771 HERMAN KARL SAALBACH, Primary Examiner M. NUSSBAUM, Assistant Examiner US. Cl. X.R, 343-725, 729, 731,

P0405) UNITED STATES PATENT OFFICE m CERTIFICATE OF CORRECTION Patent No. 3 523, 297 Dated August 4 1970 Inventor(s) Maurice e It is certified that error appears in the aboveidentified patent and that said Letters Patent are hereby corrected as shown below:

E51. 1, line 58, "different" should be -difficult. "I

Col. 4, line 37, "spray" should be --array-.

QIGIED Mu REM.

U Attest:

Edward M. Fletcher, Ir. HIE-LIAM E SGHUYLER, JR-

Ll" Officer Oomissioner of Patents J 

